The present invention relates to a semiconductor light-emitting device to be used for a light source in optical communication in the 1.3 to 1.5 μm wavelength band which uses a glass fiber as a transmission medium and a semiconductor light-emitting device as a light source in the visible region.
For example, currently, in general, in optical communication using a silica-based glass fiber as a transmission medium, the 1.3 to 1.5 μm wavelength band is mainly used. As a light source for such optical communication, a semiconductor laser formed from a crystal made of elements such as In, Ga, Al, As, and P is generally used. InXGa1−XAsYP1−Y (0<X≦1, 0≦Y<1) (to be referred to as InGaAsP hereinafter), in particular, is often used for a semiconductor laser.
A conventional laser using an InGaAsP-based material has a problem that the characteristic temperature is as low as 40 to 60 K. This characteristic temperature is one of the important characteristics in practice. The relationship between a characteristic temperature T0 and a threshold current Ith is represented by
 Ith=I0exp(T/T0)
where T is the operating environment temperature of the laser, and I0 is the threshold current at zero absolute temperature. With low T0, when the operating environment temperature rises, the threshold current for the semiconductor laser abruptly increases. In order to operate the semiconductor laser at constant power, the current injected into the semiconductor laser must be increased compared with that at a low operating temperature. With this increase in injection current, the temperature of the semiconductor laser further rises, in addition to a rise in operating environment temperature. As the operating environment temperature rises, the temperature of the semiconductor laser undergoes positive feedback. As a result, the device temperature greatly rises. In a laser formed from an InGaAsP-based material, the characteristic temperature is low for the following two reasons.Reason 1
When a double-heterostructure, which is indispensable for the formation of a semiconductor laser, is formed, the discontinuity (ΔEc) in the conduction band in the energy band structure between a carrier confinement layer (cladding layer) and an active layer is small.
Reason 2
An InGaAsP-based material has a large Auger effect. With a large Auger effect, as the operating environment temperature rises, electrons in the conduction band in an active layer provide their energy to the lattice of crystal and cause non-radiative recombination with holes. A phenomenon similar to that of electrons occurs with respect to holes in the valence band. In a zinc-blende structure which is an InGaAsP-based crystal structure, since there is no energy difference between a heavy hole and a light hole in the valence band at the Γ point in the band structure, i.e., there is no spin split-off energy Δsp, the Auger effect is large.
In a semiconductor laser, since the bandgap of the active layer decreases with a rise in device temperature, the emission wavelength lengthens. The following problems are caused by this wavelength change in optical communication.
First, in optical communication, the propagation speed of light in an optical fiber depends on the wavelength owing to the wavelength dispersion of the refractive index of the optical fiber. That is, the propagation speed of light in the fiber changes depending on a change in environment temperature. For this reason, as the environment temperature changes, on the transmitting side, the waveform of a pulse which has been rectangular with respect to the time axis distorts and spreads along with propagation of light. In long-distance communication, two optical pulses sent out from the transmitting side at different times overlap each other on the receiving side and cannot be discriminated from each other. In high bit-rate transmission system, since the intervals between optical pulses are small, two pulses sent out from the transmitting side at different times cannot be discriminated from each other on the receiving side due to changes in wavelength of optical pulses even in short-distance transmission. In addition, in wavelength division multiplexing communication (so-called WDM), since the intervals between operating wavelengths are as small as about 1 to 10 nm, variations in the wavelength of a light source cause cross-talk.
As described above, in optical communication, changes in the wavelength of communication light cause a serious problem, and great importance is attached to wavelength stability. For this reason, conventionally, a semiconductor laser serving as a light source is mounted on a Peltier device to perform temperature control so as to be used for actual optical communication. This makes it possible to stabilize operating temperature and ensure wavelength stability. Currently, therefore, in order to ensure the reliability of a communication system, great importance is also attached to the reliability of a temperature control means such as a Peltier device. The cost of equipping a Peltier device, the cost of a Peltier device control circuit, and the supply of power to the Peltier device increase the cost of a communication system. In addition, the presence of a Peltier device increases the number of components and causes a deterioration in the reliability of the overall communication system.
In addition, high power cannot be obtained from conventional optical communication light sources made of the materials described above. In optical communication systems, high-power light sources are required. Obviously, the higher the optical power of a light source, the longer the transmission distance. In large-capacity communication, since the pulse width of an optical signal is small, the energy of an optical pulse must be increased to realize communication with a high signal/noise ratio (S/N). In order to increase the energy of an optical pulse, a high-power light source is required.
The use of a conventional InGaAsP-based material will lead to a small band discontinuity ΔEc in the conduction band in a double-heterostructure forming a semiconductor laser. For this reason, the injection of a large current easily causes a carrier overflow. The carrier overflow is a phenomenon in which carriers pass through a light-emitting layer without radiative recombination in the light-emitting layer. FIG. 19 shows a phenomenon in which electrons serving as carriers move from an n-type carrier confinement layer into a p-type carrier confinement layer through a light-emitting layer.
As described above, in a semiconductor laser using a conventional InGaAsP-based material, the injection of a large current does not increase optical power but only generates heat. In order to solve this problem, attempts have been made to use Al-included compound semiconductor for a p-type carrier confinement layer adjacent to a light-emitting layer. This slightly increases the band discontinuity in the conduction band and improves the characteristics of a semiconductor laser. However, the optical output of a semiconductor laser based on this technique is limited to the level of 100 mW in the 1.55-μm band.
In addition to the two problems described above, a conventional InGaAsP-based material has a problem that it is very difficult to realize a device for emitting light at a wavelength of 1.48 μm at which the highest excitation efficiency can be obtained in an optical fiber amplifier in the 1.55-μm band. This is because the composition of a crystal that emits light in the 1.55-μm wavelength band exists in a thermodynamically immiscible region in the InGaAsP system. In order to acquire a crystal in an immiscible region, crystal growth must be done under a thermodynamically nonequilibrium condition. Even if a crystal in an immiscible region is obtained under a thermal nonequilibrium condition, its crystallinity is worse than that of a crystal grown under a thermal equilibrium condition. Although devices have been manufactured by using such crystals, they exhibit low optical power and low luminous efficiency, and the device lifetimes are short.
Problems in conventional light-emitting devices in the visible region will be described next. The first problem is that light-emitting diodes (to be referred to as LEDs hereinafter) for emitting blue, green, and yellow light beams and the like shorter in wavelength than a wavelength of 670 nm exhibit poor characteristics. Blue and green LEDs have been manufactured by using quantum well structures having well layers made of InGaN, and available on the market. It is, however, confirmed that phase separation occurs in well layers made of InGaN which are used for these devices. More specifically, regions (dots) with high In compositions are scattered in InGaN with a uniform In composition. Since these dots differ in size, the resultant light emission has a large spectrum width.
The second problem in the conventional light-emitting devices in the visible region is that no high-quality yellow LED has been obtained. For example, yellow LEDs have already been used for the yellow lights in some traffic signals mainly in the US. InGaAlP is used as a material for these devices. In a yellow light-emitting device using InGaAlP, however, the band discontinuity in the conduction band in the heterostructure made of InGaAlP materials with different compositions is small. For this reason, as described in association with the optical communication light sources, the efficiency of conversion from an injected current into light is low. In addition, an ineffective current in terms of light emission, generates heat, and hence the characteristics of the device degrade by this heat.
In order to solve this problem, attempts are made to manufacture yellow LEDs by using InGaN. However, light emission produced by an yellow LED using InGaN is one order of magnitude or more lower in intensity than blue and green light emissions. This is because in the yellow region, strong phase separation of InGaN occurs to result in poor crystallinity.
Another conventional problem in a light-emitting device in the visible region is that no practical semiconductor laser has been realized. Although LEDs can be manufactured in a wavelength range from blue to orange, no laser structure has been manufactured. This is because no device structure designed to oscillate at a low threshold current can be formed by using any conventional semiconductor materials. Only devices in a wavelength range longer than 630 nm have been manufactured.
Lasers in the 630-nm wavelength range have been manufactured by using InGaAlP-based materials. However, since the discontinuity ΔEc in the conduction band between a light-emitting layer and a cladding layer (carrier confinement layer) is small, an electron overflow is prevented by using an electron reflector based on electron wave interference which is formed from a multi-quantum well called a multi-quantum-barrier (commonly called an MQB) for a p-type cladding layer. The use of an MQB, however, leads to a high driving voltage for the device because of the presence of many band discontinuities in the conduction band.
In addition, the number of multi-quantum well layers in an active layer is one to three in general, whereas about several ten multi-quantum well layers are required in an MQB. The precision of the thickness of each quantum well layer in an active layer corresponds to the precision associated with the wavelength of light in the multi-quantum well. In contrast to this, in an MQB, this precision corresponds to the precision associated with an electron wave shorter in wavelength than light, and hence stricter thickness precision is required. In practice, the precision of the thickness of a quantum well layer is about 1/10 nm. As described above, according to the above conventional devices, an MQB is difficult to manufacture compared with a multi-quantum well serving as an active layer of a light-emitting device.